There is theory, then there is empirical evidence. Both are equally substantial, in my opinion. However, I personally don't think that two such subjects should be used to argue against each others case, rather they should be used to benefit one another.

Even so, I don't know much about phone hardware. My experience of hardware comes from old school computers (pentium 1, 2 and 3 era, k6-2, etc) and I know that the majority of that stuff is very different than current phone hardware, at least in terms of overclocking.

However, all I can say is that in my experience, running my N900 @ 805mhz (no voltage changes) has been rock solid stable. That being said, I have enjoyed the use of my device substantially more than when it was at the stock 600mhz clock rate.

It's not a huge difference, but it is very noticeable. I feel that this way I am not as tempted to buy another device just because it is faster - as mine isn't so slow anymore. I will keep this up until the upcoming devices are phenomenal, justifying my need to move on to something better. That being said, I personally feel that overclocking my own device is in fact an investment - it's preventing me from getting bored and wasting my money on the next coolest phone prematurely, as there will inevitably be something even cooler than that just over the horizon.

In the end, we all know that the OMAP 3430 is designed to be scalable at up to 1ghz. Although there are more factors than the cpu alone that contribute in the cpu's clock rate, at least we know that keeping below 1ghz isn't really pushing the envelope that much. This is probably the biggest reason why we, on this forum, haven't read many (if any, i haven't at least) horror stories of overclocking too much where their device turns into a brick.

Either way, to answer the OP's question (and I hope we all can agree on this) - YES, the N900 is overclockable - even relatively safer than overclocking your E90. However, that is not to say that there is no risk, it's just very very minimal.

PS - Feel free to send your E90 to me. I've always wanted that phone, but I went for a US spec N95-8gb instead because I wanted 3g in the states. The E90 was WAY cooler than my dumpy N95 that kept falling apart. Hah.

Good luck and I hope this helps! I've loved my N900, however I do miss some of the phone functionality with Symbian S60 (even though the N900 is still good enough), and I am very bummed out that we didn't get Flash 10.1 and we most likely never will. Boo. However, the Maemo5 OS is fantastic for what I like to use my N900 for - basically being almost a mini laptop. Hah.